1. Field of the Invention
The present invention relates to: a polarization conversion element for rotating by 90 degrees the polarization direction of linear polarized light having a fixed polarization direction; a polarized light illumination optical element for converting non-polarized illumination light into linear polarized light having a fixed polarization direction; and a liquid crystal projector whose illumination optical system employs these elements.
2. Description of Related Art
Various kinds of liquid crystal projectors are commercially available in which an image displayed on a liquid crystal display panel is illuminated by light from a light source lamp and then projected onto a screen. As known well, a liquid crystal display panel has: a liquid crystal layer of a thickness that encloses liquid crystal molecules; and a polarizer and an analyzer arranged on the incident surface side and the exit surface side, respectively. The polarizer and the analyzer are arranged in such a manner that their polarization directions are perpendicular or parallel to each other. Then, passage of linear polarized entering the liquid crystal layer is controlled in accordance with the orientation directions of the liquid crystal molecules, so that the amount of linear polarized light emitted through the analyzer is adjusted.
In the illumination optical system of such a liquid crystal projector, in general, a polarized light illumination optical element is employed that converts non-polarized light from a light source into linear polarized light having the same polarization direction as that of the polarizer of the liquid crystal display panel. As an example of this polarized light illumination optical element, as described in JP-A-2001-235624, a prism array is widely employed that is constructed from a combination of a polarization beam splitter and a ½-wavelength plate. Such a polarization beam splitter has a polarized light separation surface that transmits one of the two linear polarized light components whose polarization directions are perpendicular to each other, and that reflects the other component. Then, the polarization direction of one of the two linear polarized light components separated by this polarized light separation surface is rotated by 90 degrees through the ½-wavelength plate. After that, this linear polarized light component is mixed with the other linear polarized light component so that a linear polarized light component having an aligned polarization direction is obtained.
In a polarized light illumination optical element constructed from a combination of a polarization beam splitter and a ½-wavelength plate, when a glass prism or alternatively a heat resistive plastic material is employed for the polarization beam splitter while an inorganic dielectric multilayer film is employed for the polarized light separation surface, satisfactory heat resistance is ensured for these components. Nevertheless, since an organic sheet film is employed for the ½-wavelength plate, fading and a degradation in the polarization conversion efficiency are often arise when the optical element is used near a light source for a long time. For the purpose of heat resistance improvement, a crystalline material as such quartz having birefringence is proposed to be employed for the ½-wavelength plate. Nevertheless, the crystal itself is expensive. Further, the crystal need be machined under precise control of the crystalline optical axes. These situations cause an increase in the fabrication cost.